It is well known that cardiac arrhythmias such as ventricular fibrillation may be controlled with devices such as implantable defibrillators. Many different types of defibrillation electrodes have been suggested over the years, as can be seen from the following examples. In this discussion, no distinction will be made between cardioversion and defibrillation; both will be referred to as defibrillation.
U.S. Pat. No. 3,942,536 issued to Mirowski et al. discloses an intravascular bipolar catheter electrode system wherein each of two electrodes is composed of a plurality of spaced, low impedance rings. As implanted, the first electrode is located within the right ventricle (RV) and the second electrode is located in the superior vena cava (SVC).
In U.S. Pat. No. 4,161,952 issued to Kinney et al., a catheter electrode has a coil of wound spring wire, with filler material beneath and between individual turns of coil such that only the outside of the wound wire is exposed to the patient's body. It is designed to reside in or about the heart, as in the SVC or in the coronary sinus (CS).
U.S. Pat. No. 4,922,927 issued to Fine et al. teaches the use of tightly wound wire forming a tight coil on a support that is flared to provide a greater diameter along its midsection than at its ends, to form an RV electrode. A copper-zirconium alloy wrapped with tantalum and coated with iridium oxide is suggested for the tightly wound wire.
Other types of transvenously placed leads are disclosed in U.S. Pat. No. 4,998,975 issued to Cohen et al. One lead is placed through the heart wall, and into the pericardial space, and another is placed endocardially in a conventional manner. Both leads are shown with several embodiments, with the examples of general electrode construction being to expose a section of the conductor coil, or to use ring electrodes similar to those used in conventional bipolar pacemaker leads. Cohen et al. also describe two methods for steering more current to a selected region of the heart. The first method is to apply various voltages to the connectors of each of four electrodes. The second method uses the resistance of conductors, both between connector and electrode, and between two electrodes on the same lead, and the body tissue resistance between electrodes on different leads, to form a voltage divider, thus creating a different potential at each electrode.
Another lead system patent, U.S. Pat. No. 5,007,436 issued to Smits, describes electrodes of both J and straight configurations, for use in the RV, right atrium, great cardiac vein, or CS. The fabrication methods suggested use close wound conductive coils mounted exterior to an elongated insulative sheath, or the method of Kinney et al.
Spiral shaped electrodes for endocardial, epicardial, or extrapericardial implantation are described in Heil, Jr. et al., U.S. Pat. No. 5,016,808, Fogarty et al., U.S. Pat. No. 4,860,769, and Hauser et al., U.S. Pat. No. 5,052,407. The electrodes of these patents use various construction techniques, including electrodeposition or vapor deposition onto a plastic tube, helically wound wire (round or ribbon, unifilar or multifilar, single or double helix) or conductive rings on a flexible insulating portion, and conductive screen wrapped around a tubular body.
Other defibrillation leads are disclosed in Mehra et al., U.S. Pat. No. 5,144,960, and in Bardy et al., U.S. Pat. No. 5,174,288.
Endotak SQ Model 0048 (Cardiac Pacemakers Inc., St. Paul, Minn., USA), described in "A Subcutaneous Lead Array for Implantable Cardioverter Defibrillators" by Jordaens et al., published in PACE, Vol. 16, Jul. 1993, Part I, is an electrode system consisting of three conductive elements that can be subcutaneously inserted. The conductive elements of this "array lead" are made of electrically common multifilar coil, joined in a silicone yoke, and separately introduced with a lead tunneler and peel-away sheaths.
Epicardial defibrillation leads typically are made of wire mesh, which is welded in several places to another piece of mesh or foil, which is in turn crimped to a conductor. The mesh wire diameter is typically 0.10 min. The epicardial lead shown in Moore et al., U.S. Pat. No. 4,314,095, has an electrode connection formed by crimping a piece of wire mesh and a conductor into the channel of a U-shaped clip, then welding the mesh portion to a wire mesh electrode. Ideker et al., in U.S. Pat. No. 4,827,932, disclose a connection formed by spot welding a pair of tabs to both sides of a mesh electrode, then inserting the ends of the tabs and a coil conductor into a sleeve, then crimping the components together.
Endocardial lead electrodes for pacing and defibrillation typically are joined to conductors by crimps or welds. U.S. Pat. No. 4,662,382, issued to Sleutz et al., describes such a connection made by crimping an electrode wire and a conductor wire into a sleeve. A second electrode has a hollow portion to accept a conductor coil and crimp pin, which get crimped together. U.S. Pat. No. 4,784,161, issued to Skalsky et al., describes a crimp connection having both an electrode wire and conductor wires wrapped around a shaft, with a crimp sleeve over both. In another embodiment, a bundle of electrode wires and a helical conductor have a support pin through the middle of them; a crimp sleeve covers both the conductor and the bundled wires. U.S. Pat. Nos. 4,214,804 and 4,328, 812, to Little and Ufford et al. respectively, disclose press fit, or swage fit, connections of ring electrodes to conductor coils. U.S. Pat. No. 4,161,952, to Kinney et al., teaches the use of metal connecting pieces to which is welded a 0.76 mm diameter electrode wire. An electrically conductive polymer such as silver-filled epoxy is used to electrically connect the conductor to the metal connecting pieces.
As defibrillator technology improves and the demand for defibrillators increases, it becomes increasingly desirable to have leads available that are easily implanted and capable of withstanding repeated flexing over a long period of time. In order to provide improved flexibility, prior art systems have begun to use very small wires which are fatigue resistant. This however presents a problem of making reliable electrical connections. Crimps, swages, press fit connections, and the like require at least some deformation of the parts being connected, In the case of fine wires, say 0.08 mm diameter, it is very difficult to deform the wire to form a strong connection without weakening or breaking it. This is especially true considering that tolerances on crimp sleeves, crimp pins, and crimp tool jaws can easily add up to more than the diameter of the wire being crimped. Welds require some melting of the material being welded. Resistance welds in particular require the application of pressure. For fine wire, the wire may melt through or be crushed during the welding process. For fine coiled wire, neither crimping nor simple welding is suitable because the wire needs to be unwound before pressure is applied, since flattening a fine coil will break it. On the other hand, the coiled wire could become damaged by the process of unwinding it. Therefore, another method must be used to join these small wire electrode elements to their lead conductors.